1. Field of the Invention
The present invention relates to an optical phase locked loop that locks a locally generated optical signal in phase and frequency with a received optical signal for coherent detection in optical communication systems.
2. Description of the Related Art
The need to increase optical communication capacity has focused attention and research effort on phase modulation systems, which can assure a higher signal-to-noise ratio than is obtainable by conventional on-off keying (OOK) or amplitude shift keying (ASK). Both phase shift keying (PSK) and differential phase shift keying (DPSK) are being studied.
In any phase modulated communication system, a digital signal modulates the phase of a carrier signal. In optical PSK and DPSK systems, the carrier signal is typically an optical pulse train. In DPSK modulation, the digital information is encoded in the relative phase of consecutive pulses. For example, in a binary DPSK optical transmission signal the phase of each successive optical pulse is shifted by 0 or π radians. In PSK modulation, the digital information is encoded in the absolute phase of each transmitted pulse, so the phase of each received pulse must be compared with the phase of a reference signal.
DPSK is easier to implement than PSK, because a DPSK receiver only has to compare the phases of successive received pulses and does not have to generate a reference signal, but the DPSK demodulation process is complex and error-prone.
To generate the reference signal necessary for optical PSK, the receiver must have a local light source precisely locked in frequency and phase with the transmitted optical carrier signal. Once this requirement is met, however, the PSK demodulation process is relatively simple and error-free, because OOK intensity modulation techniques can be used.
Optical phase modulation schemes are also referred to as coherent modulation schemes, since they exploit the coherence of the optical carrier signal. In contrast, schemes that modulate optical pulse intensity make no use of the coherence of the optical carrier signal.
The receiving methods employed in known coherent optical communication systems include both heterodyne and homodyne detection, and homodyne phase diversity detection. All of these methods mix the received optical signal with a reference optical signal generated by a local light source. In heterodyne detection the frequency of the reference optical signal differs slightly from the frequency of the transmitted optical signal, and the phase of the received pulses is detected from the resulting interference beats. In homodyne detection, the reference optical signal has the same frequency as the transmitted optical signal, and the phase of the received pulses is detected as constructive or destructive interference, or varying degrees thereof. In homodyne phase diversity detection, the received signal and reference signal are mixed with a plurality of phase offsets. Homodyne and homodyne phase diversity detection require a strict phase relation to be maintained between the reference optical signal and the transmitted carrier signal.
Among the many optical communication systems that have been reported, there are some that use an optical phase locked loop to maintain the necessary phase relationship between the received and reference optical signals. One such system is described by Ito et al. in Japanese Patent Application Publication No. H07-046191.
An optical phase locked loop operates by the same principle as an electrical phase locked loop (PLL). Most reported optical phase locked loops, however, directly modulate the frequency of a local laser with an optical signal including the phase error, as described, for example, by Imayado et al. in Japanese Patent Application Publication No. 2000-68580. This phase locking system leads to expensive fabrication because of the large size of the necessary components. Furthermore, it sometimes requires direct frequency modulation of a high-speed electrical signal (e.g., several tens of gigahertz), which is difficult because of the limited operating speed of electronic components.
Recently, optical phase locked loops employing sideband techniques have also been developed, as described, for example, by Camatel et al. in ‘10 Gbit/s 2-PSK transmission and homodyne coherent detection using commercial optical components’, ECOC 2003, Vol. 3, We. P. 122, pp. 800-801. The method described by Camatel et al. performs intensity modulation on a locally generated optical signal and selects one of the resulting sidebands as a reference signal; another method uses a bandpass filter to select part of a sideband generated by a phase modulator. Interference between the received optical signal and the reference signal provides feedback for phase locking.
A problem with these conventional optical phase locked loops is that they use the locally generated optical signal inefficiently. Since they use only one sideband resulting from modulation of the locally generated optical signal, they discard most of the energy of the locally generated signal. To compensate for the inefficiency, the selected sideband may require amplification by an optical amplifier, but then noise arising during the amplification process degrades the quality of the reference signal.
Another problem is frequency jitter in the received optical signal and the locally generated optical signal. To cope with such jitter, the optical phase locked loop may have to operate over a frequency range that exceeds the operating range of its electronic components. Keeping the jitter of the optical signals within the operating range of the electronic components requires expensive frequency control equipment. Electronic components with frequency ranges wide enough to accommodate normal optical frequency jitter are also expensive, however.